The present invention relates to resistance heaters and more specifically to a power adapter for resistance heaters used on injection molding runner nozzles and to a method of termination that allows said power adapters to be used for thick film resistance heaters.
It is well-known in the plastics industry to mold thermoplastics with hot runner systems. In this system, melted thermoplastics (xe2x80x9cmeltxe2x80x9d) is run through a series of channels and through a series of nozzles before being injected into a mold. The nozzles protrude from a manifold, which houses the melt channels. A pair of complementing mold platens together form a mold cavity into which the nozzles empty the melt.
The importance of keeping the nozzles heated to prevent the melt from freezing before entering the mold cavity is also well-known. Originally, resistive element heaters were integrally designed with the nozzles. However, the heating elements of these heaters had shorter life spans than the nozzles, so it was unnecessarily expensive to replace the entire nozzle when the heating element failed.
To overcome this expense, heaters were built separate from the nozzle. For example, cable heaters would be embedded in or assembled with a band or ring that would clamp or slide over the nozzle. More recently it has been proposed by Watlow Electric Manufacturing Company of St. Louis, Mo., to place a thick film resistive heating element on a metal substrate that could be slid over a nozzle. Obviously, if the substrate is metal, a dielectric layer must also be introduced between the heating element and the substrate. This is typically done by a spraying, printing, or dipping method, or any other similar method, all of which are well-known and equivalent alternatives to one of ordinary skill in the art of thick film technology.
The major limitation, even with the separate heaters, is that changing the heater requires significant down time. The heaters are wired into the molding system, which has wiring channels embedded in the manifold. Therefore, replacing the heater means accessing the wiring channels, which in turn means a significant down time for a heater change.
The other limitation related specifically to the thick film heaters is that there was no easy way to terminate leads on the end of the heating elements. Power leads on thick film heaters have been typically soldered to the heating element. The limitation here is that solder melts at 190xc2x0 C. so the application temperature is limited to approximately 125xc2x0 C., which is insufficient for a hot runner system.
It is also known to use mechanical terminations with a thick film heater where a ring with an insulator (such as mica) is clamped over the leads to mechanically hold them in place against the heating element. There are many disadvantages to this type of assembly. For example, the assembly is bulky, cumbersome, complex, and expensive. Even worse, however, the additional space taken by this assembly creates a thermal mass, which can increase temperature variations of the heater.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art by providing an electrical adapter to provide power to a nozzle heater for a hot runner molding system.
It is another object of the present invention to provide such an adapter that allows for quick and easy removal and replacement of a failed heater.
It is yet another object of the present invention to provide such an adapter to work with a heater that uses a thick film heating element.
It is still another object of the present invention to provide a new termination of thick film heating elements that can withstand high temperatures.
It is still further an object of the present invention to provide such a termination of thick film heating elements that is compact, yet durable.
In keeping with the above enumerated objects, the present invention provides a power adapter that is permanently (or semi-permanently) installed in an injection molding system. The power adapter allows insertion of a heater and has a mechanism to retain the heater by rotating the heater into a locked position. Conversely, the heater may be removed, upon failure for instance, simply by rotating the heater in the opposite direction (to unlock it from the retaining mechanism) and pulling the heater off the nozzle.
In a hot runner system the nozzles are kept hotter than the mold. In a cold runner system (a similar xe2x80x9crunnerlessxe2x80x9d injection molding system) the nozzles may be heated, but not to as a high a degree as the mold. The present system was designed for hot runner systems, but works equally well with cold runner systems without departing from the objects of the invention.
In another aspect of the invention electrical contacts are embedded in the adapter to provide power to the heater. The contacts are preferably pliant and are electrically hard-wired through the wiring channels of the manifold.
In yet another aspect of the present invention, a thick film heater is provided with terminal plates that are designed to engage the contacts located inside the power adapter. The preferred heater is a thick film heating element deposited on a metal substrate. A novel method is used to affix the terminal plates to the end of the thick film heating element, which involves using a conductive silver-based paste as an adhesive between the terminal pad and the heating element. This termination method, which makes the present invention practical can be used in a variety of other applications unrelated to the power adapter.